Lithium silicate glass ceramic and glass with monovalent metal oxide

ABSTRACT

Lithium silicate glass ceramics and glasses comprising specific oxides of monovalent elements are described which crystallize at low temperatures and are suitable in particular as dental materials.

This present application claims priority to and is a continuationapplication of U.S. application Ser. No. 14/001,180, filed Dec. 2, 2013,which is a National Stage of International patent applicationPCT/EP2012/070219, filed on Oct. 11, 2012, which claims priority toEuropean patent application No. 11185334.7 filed on Oct. 14, 2011, thedisclosures of which are incorporated herein by reference in theirentirety.

The invention relates to lithium silicate glass ceramic and glass whichcomprise monovalent metal oxide selected from Rb₂O, Cs₂O and mixturesthereof and are particularly suitable for use in dentistry, preferablyfor the preparation of dental restorations.

Lithium silicate glass ceramics are characterized as a rule by very goodmechanical properties, which is why they have been used for a long timein the dental field and there primarily for the preparation of dentalcrowns and small bridges. The known lithium silicate glass ceramicsusually contain as main components SiO₂, Li₂O, Al₂O₃, Na₂O or K₂O, andnucleating agents such as P₂O₅.

DE 24 51 121 describes lithium disilicate glass ceramics which containK₂O and Al₂O₃. They are prepared from corresponding nuclei-containingstarting glasses which are heated to temperatures of from 850 to 870° C.for the crystallization of lithium disilicate.

EP 827 941 describes sinterable lithium disilicate glass ceramics fordental purposes, which also contain K₂O or Na₂O in addition to La₂O₃.The lithium disilicate crystal phase is produced at a temperature of850° C.

Lithium disilicate glass ceramics which contain K₂O and Al₂O₃ are knownfrom EP 916 625. A heat treatment is carried out at 870° C. for theformation of lithium disilicate.

EP 1 505 041 describes lithium silicate glass ceramics containing K₂Oand Al₂O₃, which, when lithium metasilicate is present as main crystalphase, can be very satisfactorily machined e.g. by means of CAD/CAMprocesses, in order to then be converted by further heat treatment attemperatures of from 830 to 850° C. into high-strength lithiumdisilicate glass ceramics.

EP 1 688 398 describes similar K₂O— and Al₂O₃-containing lithiumsilicate glass ceramics which additionally are substantially free fromZnO. A heat treatment at 830 to 880° C. is applied to them to producelithium disilicate.

U.S. Pat. No. 5,507,981 describes processes for producing dentalrestorations and glass ceramics that can be used in these processes.These are in particular lithium disilicate glass ceramics which containAl₂O₃ and usually either Na₂O or K₂O.

U.S. Pat. No. 6,455,451 relates to lithium disilicate glass ceramicswhich can also comprise Cs₂O in specific embodiments. However, in theseembodiments the presence of significant quantities of Al₂O and BaO isalso necessary. The production of the desired lithium disilicate crystalphase requires high temperatures of from 800 to 1000° C.

WO 2008/106958 discloses lithium disilicate glass ceramics for veneeringzirconium oxide ceramics. The glass ceramics contain Na₂O and areproduced by heat treatment of nuclei-containing glasses at 800 to 940°C.

WO 2009/126317 describes GeO₂-containing lithium metasilicate glassceramics which also comprise K₂O and Al₂O₃. The glass ceramics areprocessed to form dental products primarily by machining.

WO 2011/076422 relates to lithium disilicate glass ceramics which alsocomprise K₂O and Al₂O₃ in addition to high levels of ZrO₂ or HfO₂. Thecrystallization of lithium disilicate takes place at temperatures offrom 800 to 1040° C.

The known lithium disilicate glass ceramics have in common that theyrequire heat treatments at more than 800° C. in order to effect theprecipitation of lithium disilicate as main crystal phase. A largequantity of energy is therefore necessary to prepare them. Further, inthe known glass ceramics the alkali metal oxides K₂O or Na₂O, as well asAl₂O₃ and BaO, are as a rule present as essential components whichapparently are required for the production of the glass ceramics and inparticular the formation of the desired lithium disilicate main crystalphase.

There is therefore a need for lithium silicate glass ceramics during thepreparation of which the crystallization of lithium disilicate can beeffected at lower temperatures. Further, it should also be possible toprepare them without the alkali metal oxides K₂O or Na₂O as well asAl₂O₃ and BaO, previously regarded as necessary, and they should besuitable in particular for the preparation of dental restorationsprimarily in view of their optical and mechanical properties.

This object is achieved by the lithium silicate glass ceramic accordingto any one of claim 1 to 15 or 18. Also a subject of the invention arethe starting glass according to claim 16 or 18, the lithium silicateglass with nuclei according to claims 17 and 18, the process for thepreparation of the glass ceramic and the lithium silicate glass withnuclei according to claims 19 and 20 as well as the use according toclaims 21 and 22.

The lithium silicate glass ceramic according to the invention ischaracterized in that it comprises monovalent metal oxide selected fromRb₂O, Cs₂O and mixtures thereof.

It is preferred that the glass ceramic comprises the monovalent metaloxide or mixtures thereof in an amount of 0.1 to 17.0, in particular 1.0to 15.0 and particularly preferably 1.5 to 8.0 wt.-%.

It is particularly surprising that the formation of the glass ceramicaccording to the invention with lithium disilicate as main crystal phaseis also achieved in the absence of various components regarded asnecessary for conventional glass ceramics, such as in particular K₂O,Na₂O as well as Al₂O₃ and BaO, even at very low and thus advantageouscrystallization temperatures of about 700° C.

The glass ceramic according to the invention therefore preferablycomprises less than 1.0, in particular less than 0.5 wt.-%, preferablyless than 0.1 wt.-% K₂O. The glass ceramic is particularly preferablyessentially free of K₂O.

Further, a glass ceramic is preferred which comprises K₂O, Na₂O andmixtures thereof in an amount of less than 1.0, in particular less than0.5 and preferably less than 0.1 wt.-% and particularly preferably whichis essentially free of K₂O and Na₂O.

Moreover, a glass ceramic which comprises less than 5.3, in particularless than 5.1, preferably less than 4.0 and particularly preferably lessthan 3.0 wt.-% Al₂O₃ is preferred. In a further preferred embodiment,the glass ceramic is essentially free of Al₂O₃.

In a further preferred embodiment, the glass ceramic comprises less than3.8, in particular less than 3.6 and preferably less than 2.5 wt.-% BaO.The glass ceramic is particularly preferably essentially free of BaO.

A glass ceramic is also preferred which excludes lithium silicate glassceramic comprising at least 6.1 wt.-% ZrO₂.

Further, a glass ceramic is also preferred which excludes lithiumsilicate glass ceramic comprising at least 8.5 wt.-% transition metaloxide selected from the group consisting of oxides of yttrium, oxides oftransition metals with an atomic number of 41 to 79 and mixtures ofthese oxides.

The glass ceramic according to the invention preferably comprises 55.0to 85.0, in particular 60.0 to 78.0 and preferably 62.0 to 77.0 wt.-%SiO₂.

It is also preferred that the glass ceramic comprises 9.0 to 20.0, inparticular 9.0 to 17.0 and particularly preferably 12.0 to 16.0 wt.-%Li₂O.

It is further preferred that the molar ratio between SiO₂ and Li₂O isfrom 2.2 to 2.6, in particular from 2.3 to 2.5 and particularlypreferably is about 2.4.

The glass ceramic according to the invention can also comprise anucleating agent. It is preferred that a nucleating agent is present.P₂O₅ is particularly preferably used for this. The glass ceramicpreferably comprises 0 to 12.0, in particular 1.0 to 12.0, preferably2.0 to 9.0 and particularly preferably 2.5 to 7.5 wt.-% P₂O₅.

In a further preferred embodiment, the glass ceramic comprises at leastone and preferably all of the following components:

Component wt.-% SiO₂ 55.0 to 85.0  Li₂O 9.0 to 17.0 Rb₂O and/or Cs₂O 0.1to 15.0 P₂O₅ 0 to 12.0, preferably 1.0 to 12.0.

The glass ceramic according to the invention can moreover also compriseadditional components which are selected in particular from oxides ofdivalent elements, oxides of trivalent elements, further oxides oftetravalent elements, further oxides of pentavalent elements, oxides ofhexavalent elements, melt accelerators, colourants and fluorescentagents.

In particular, the alkaline earth metal oxides, preferably CaO, BaO,MgO, SrO or a mixture thereof and particularly preferably MgO aresuitable oxides of divalent elements.

Suitable oxides of trivalent elements are in particular Y₂O₃. La₂O₃,Bi₂O₃ and mixtures thereof, and preferably Y₂O₃.

The term “further oxides of tetravalent elements” refers to oxides oftetravalent elements with the exception of SiO₂. Examples of suitablefurther oxides of tetravalent elements are TiO₂, SnO₂ and GeO₂.

The term “further oxides of pentavalent elements” refers to oxides ofpentavalent elements with the exception of P₂O₅. Examples of suitablefurther oxides of pentavalent elements are Ta₂O₅ or Nb₂O₅.

Examples of suitable oxides of hexavalent elements are WO₃ and MoO₃.

A glass ceramic is preferred which comprises at least one oxide ofdivalent elements, at least one oxide of trivalent elements, at leastone further oxide of tetravalent elements, at least one further oxide ofpentavalent elements and/or at least one oxide of hexavalent elements.

Examples of melt accelerators are fluorides.

Examples of colourants and fluorescent agents are oxides of d- andf-elements, such as the oxides of Ti, V, Sc, Mn, Fe, Co, Ta, W, Ce, Pr,Nd, Tb, Er, Dy, Gd, Eu and Yb. Metal colloids, e.g. of Ag, Au and Pd,can also be used as colourants and in addition can also act asnucleating agents. These metal colloids can be formed e.g. by reductionof corresponding oxides, chlorides or nitrates during the melting andcrystallization processes. The metal colloids are preferably present inthe glass ceramic in an amount of 0.005 to 0.5 wt.-%.

In particular, the glass ceramic according to the invention comprisesAg₂O in an amount of 0.005 to 0.5 wt.-%.

The term “main crystal phase” used below refers to the crystal phasewhich has the highest proportion by volume compared with other crystalphases.

In one embodiment, the glass ceramic according to the inventioncomprises lithium metasilicate as main crystal phase. In particular theglass ceramic comprises more than 5 vol.-%, preferably more than 10vol.-% and particularly preferably more than 15 vol.-% lithiummetasilicate crystals, relative to the total glass ceramic.

In a further particularly preferred embodiment, the glass ceramiccomprises lithium disilicate as main crystal phase. In particular theglass ceramic comprises more than 10 vol.-%, preferably more than 20vol.-% and particularly preferably more than 30 vol.-% lithiumdisilicate crystals, relative to the total glass ceramic.

The lithium disilicate glass ceramic according to the invention ischaracterized by particularly good mechanical properties and can beproduced e.g. by heat treatment of the lithium metasilicate glassceramic according to the invention. However, it can be formed inparticular by heat treatment of a corresponding starting glass or of acorresponding lithium silicate glass with nuclei.

It has surprisingly been shown that the lithium disilicate glass ceramicaccording to the invention has very good mechanical and opticalproperties even in the absence of components regarded as essential forconventional glass ceramics. The combination of its properties evenallows it to be used as dental material and in particular as materialfor the preparation of dental restorations.

The lithium disilicate glass ceramic according to the invention has inparticular a fracture toughness, measured as K_(IC) value, of at leastabout 2.0 MPa·m^(0.5) and in particular at least about 2.3 MPa·m^(0.5).This value was determined using the Vickers method and calculated usingNiihara's equation. Further, it has a high biaxial flexural strength ofpreferably from 400 to 700 MPa. Moreover, it displays a high chemicalresistance ascertained by mass loss after storage in acetic acid. Thechemical resistance is in particular less than 100 μg/cm². The biaxialflexural strength and the chemical resistance were determined accordingto ISO 6872 (2008).

The invention also relates to a lithium silicate glass with nuclei thatare suitable for forming lithium metasilicate and/or lithium disilicatecrystals, wherein the glass comprises the components of theabove-described glass ceramics according to the invention. Thus thisglass comprises monovalent metal oxide selected from Rb₂O, Cs₂O andmixtures thereof. For preferred embodiments of this glass reference ismade to the preferred embodiments described above of the glass ceramicsaccording to the invention.

The glass with nuclei according to the invention can be produced by heattreatment of a correspondingly composed starting glass according to theinvention. The lithium metasilicate glass ceramic according to theinvention can then be formed by a further heat treatment, and in turn beconverted into the lithium disilicate glass ceramic according to theinvention by further heat treatment, or the lithium disilicate glassceramic according to the invention can also preferably be formeddirectly from the glass with nuclei. The starting glass, the glass withnuclei and the lithium metasilicate glass ceramic can consequently beregarded as precursors for the production of the high-strength lithiumdisilicate glass ceramic.

The glass ceramics according to the invention and the glasses accordingto the invention are present in particular in the form of powders,granulates or blanks, e.g. monolithic blanks, such as platelets, cuboidsor cylinders, or powder compacts, in unsintered, partly sintered ordense-sintered form. They can easily be further processed in theseforms. They can, however, also be present in the form of dentalrestorations, such as inlays, onlays, crowns, veneers, facets orabutments.

The invention also relates to a process for the preparation of the glassceramic according to the invention and the glass with nuclei accordingto the invention, in which a correspondingly composed starting glass,the glass with nuclei according to the invention or the lithiummetasilicate glass ceramic according to the invention is subjected to atleast one heat treatment in the range of 450 to 950° C., in particular450 to 800 and preferably 450 to 750° C.

The starting glass according to the invention therefore comprisesmonovalent metal oxide selected from Rb₂O, Cs₂O and mixtures thereof. Inaddition, it preferably also comprises suitable amounts of SiO₂ and Li₂Oin order to allow the formation of a lithium silicate glass ceramic, andin particular a lithium disilicate glass ceramic. Moreover, the startingglass can also comprise still further components, such as are givenabove for the lithium silicate glass ceramic according to the invention.All those embodiments are preferred for the starting glass which arealso given as preferred for the glass ceramic according to theinvention.

In the process according to the invention, the glass with nuclei isusually prepared by means of a heat treatment of the starting glass at atemperature of in particular 480 to 560° C. The lithium disilicate glassceramic according to the invention is then preferably produced from theglass with nuclei through further heat treatment at usually 600 to 750and in particular 650 to 750° C.

Thus, much lower temperatures are used according to the invention forthe crystallization of lithium disilicate than with the conventionallithium disilicate glass ceramics. The energy thus saved represents aclear advantage. Surprisingly, this low crystallization temperature isalso possible in the absence of components, such as K₂O and Al₂O₂ aswell as BaO, regarded as essential for conventional glass ceramics.

To prepare the starting glass, the procedure is in particular that amixture of suitable starting materials, such as carbonates, oxides,phosphates and fluorides, is melted at temperatures of in particularfrom 1300 to 1600° C. for 2 to 10 h. To achieve a particularly highhomogeneity, the obtained glass melt is poured into water in order toform a glass granulate, and the obtained glass granulate is then meltedagain.

The melt can then be poured into moulds to produce blanks of thestarting glass, so-called solid glass blanks or monolithic blanks.

It is also possible to put the melt into water again in order to preparea granulate. This granulate can then be pressed, after grinding andoptionally addition of further components, such as colourants andfluorescent agents, to form a blank, a so-called powder compact.

Finally, the starting glass can also be processed to form a powder aftergranulation.

The starting glass, e.g. in the form of a solid glass blank, a powdercompact or in the form of a powder, is then subjected to at least oneheat treatment in the range of 450 to 950° C. It is preferred that afirst heat treatment is initially carried out at a temperature in therange of 480 to 560° C. to prepare a glass according to the inventionwith nuclei which are suitable for forming lithium metasilicate and/orlithium disilicate crystals. This first heat treatment is preferablycarried out for a period of 10 min to 120 min and in particular 10 minto 30 min. The glass with nuclei can then preferably be subjected to atleast one further heat treatment at a higher temperature and inparticular more than 570° C. to effect crystallization of lithiummetasilicate or lithium disilicate. This further heat treatment ispreferably carried out for a period of from 10 min to 120 min, inparticular 10 min to 60 min and particularly preferably 10 min to 30min. To crystallize lithium disilicate, the further heat treatment isusually carried out at 600 to 750, preferably 650 to 750 and quiteparticularly preferably 700 to 750° C.

In a preferred embodiment of the process, therefore

-   -   (a) the starting glass is subjected to a heat treatment at a        temperature of 480 to 560° C. in order to form the glass with        nuclei, and    -   (b) the glass with nuclei is subjected to a heat treatment at a        temperature of 700 to 750° C. in order to form the glass ceramic        with lithium disilicate as main crystal phase.

The duration of the heat treatments carried out in (a) and (b) ispreferably as given above.

The at least one heat treatment carried out in the process according tothe invention can also take place during hot pressing or sintering-on ofthe glass according to the invention or the glass ceramic according tothe invention.

Dental restorations, such as bridges, inlays, onlays, crowns, veneers,facets or abutments, can be prepared from the glass ceramics accordingto the invention and the glasses according to the invention. Theinvention therefore also relates to their use for the preparation ofdental restorations. It is preferred that the glass ceramic or the glassis shaped into the desired dental restoration by pressing or machining.

The pressing is usually carried out under increased pressure andincreased temperature. It is preferred that the pressing is carried outat a temperature of 700 to 1200° C. It is further preferred to carry outthe pressing at a pressure of 2 to 10 bar. During pressing, the desiredshape change is achieved by viscous flow of the material used. Thestarting glass according to the invention and in particular the glasswith nuclei according to the invention, the lithium metasilicate glassceramic according to the invention and the lithium disilicate glassceramic according to the invention can be used for the pressing. Theglasses and glass ceramics according to the invention can be used inparticular in the form of blanks, e.g. solid blanks or powder compacts,e.g. in unsintered, partly sintered or dense-sintered form.

The machining is usually carried out by material removal processes andin particular by milling and/or grinding. It is particularly preferredthat the machining is carried out within the framework of a CAD/CAMprocess. The starting glass according to the invention, the glass withnuclei according to the invention, the lithium metasilicate glassceramic according to the invention and the lithium disilicate glassceramic according to the invention can be used for the machining. Theglasses and glass ceramics according to the invention can be used inparticular in the form of blanks, e.g. solid blanks or powder compacts,e.g. in unsintered, partly sintered or dense-sintered form. The lithiummetasilicate glass ceramic according to the invention and lithiumdisilicate glass ceramic according to the invention are preferably usedfor the machining. The lithium disilicate glass ceramic can also be usedin a not fully crystallized form which was produced by heat treatment ata lower temperature. This has the advantage that an easier machining,and thus the use of simpler equipment for the machining, is possible.After the machining of such a partly crystallized material, the latteris usually subjected to a heat treatment at a higher temperature and inparticular 650 to 750° C. and preferably about 700° C. in order toeffect further crystallization of lithium disilicate.

In general, after the preparation of the dental restoration shaped asdesired by pressing or machining, it can also in particular beheat-treated in order to convert the precursors used, such as startingglass, glass with nuclei or lithium metasilicate glass ceramic, intolithium disilicate glass ceramic or to increase the crystallization oflithium disilicate or to reduce the porosity, e.g. of a porous powdercompact used.

However, the glass ceramic according to the invention and the glassaccording to the invention are also suitable as coating material of e.g.ceramics and glass ceramics. The invention is therefore also directedtowards the use of the glass according to the invention or the glassceramic according to the invention for coating in particular ceramicsand glass ceramics.

The invention also relates to a process for coating ceramics and glassceramics, in which the glass ceramic according to the invention or theglass according to the invention is applied to the ceramic or glassceramic and is exposed to increased temperature.

This can take place in particular by sintering-on and preferably bypressing-on. With sintering-on, the glass ceramic or the glass isapplied to the material to be coated, such as ceramic or glass ceramic,in the usual way, e.g. as powder, and then sintered at increasedtemperature. With the preferred pressing-on, the glass ceramic accordingto the invention or the glass according to the invention is pressed on,e.g. in the form of powder compacts or monolithic blanks, at anincreased temperature of e.g. 700 to 1200° C., applying pressure, e.g. 2to bar. The methods described in EP 231 773 and the press furnacedisclosed therein can be used in particular for this. A suitable furnaceis e.g. the Programat EP 5000 from Ivoclar Vivadent AG, Liechtenstein.

It is preferred that, after conclusion of the coating process, the glassceramic according to the invention is present with lithium disilicate asmain crystal phase, as such glass ceramic has particularly goodproperties.

Because of the above-described properties of the glass ceramic accordingto the invention and the glass according to the invention as itsprecursor, they are suitable in particular for use in dentistry. Asubject of the invention is therefore also the use of the glass ceramicaccording to the invention or the glass according to the invention as adental material and in particular for the preparation of dentalrestorations or as a coating material for dental restorations, such ascrowns, bridges and abutments.

Finally, the glasses and glass ceramics according to the invention canalso be mixed together with other glasses and glass ceramics in order toproduce dental materials with properties set as desired. Compositionsand in particular dental materials which comprise the glass according tothe invention or the glass ceramic according to the invention incombination with at least one other glass and/or one other glass ceramictherefore represent a further subject of the invention. The glassaccording to the invention or the glass ceramic according to theinvention can therefore be used in particular as main component of aninorganic-inorganic composite or in combination with a plurality ofother glasses and/or glass ceramics, wherein the composites orcombinations can be used in particular as dental materials. Thecombinations or composites can particularly preferably be present in theform of sintered blanks. Examples of other glasses and glass ceramicsfor the preparation of inorganic-inorganic composites and ofcombinations are disclosed in DE 43 14 817, DE 44 23 793, DE 44 23 794,DE 44 28 839, DE 196 47 739, DE 197 25 553, DE 197 25 555, DE 100 31 431and DE 10 2007 011 337. These glasses and glass ceramics belong to thesilicate, borate, phosphate or aluminosilicate group. Preferred glassesand glass ceramics are of SiO₂-Al₂O₃-K₂O type (with cubic or tetragonalleucite crystals), SiO₂-B₂O₃-Na₂O type, alkali-silicate type,alkali-zinc-silicate type, silicophosphate type, SiO₂-ZrO₂ type and/orlithium-aluminosilicate type (with spodumene crystals). By mixing suchglasses or glass ceramics with the glasses and/or glass ceramicsaccording to the invention, for example the coefficient of thermalexpansion can be set as desired in a broad range of from 6 to 20·10⁻⁶K⁻¹.

The invention is explained in more detail below by means of examples.

EXAMPLES Examples 1 to 16 Composition and Crystal Phases

A total of 16 glasses and glass ceramics according to the invention withthe composition given in Table I were prepared by melting correspondingstarting glasses followed by heat treatment for controlled nucleationand crystallization.

For this, the starting glasses weighing from 100 to 200 g were firstmelted from customary raw materials at 1400 to 1500° C., wherein themelting was very easily possible without formation of bubbles orstreaks. By pouring the starting glasses into water, glass frits wereprepared which were then melted a second time at 1450 to 1550° C. for 1to 3 h for homogenization.

In the case of Examples 1 to 9 and 11 to 16, the obtained glass meltswere then poured into preheated moulds in order to produce glassmonoliths. All glass monoliths proved transparent.

In the case of Example 10, the obtained glass melt was cooled to 1400°C. and converted to a finely divided granulate by pouring into water.The granulate was dried and ground to a powder with a particle size of<90 μm. This powder was moistened with some water and moulded to form apowder compact at a pressure of 20 MPa.

The glass monoliths (Examples 1-9 and 11-16) as well as the powdercompact (Example 10) were then converted by thermal treatment to glassesand glass ceramics according to the invention. The thermal treatmentsused for controlled nucleation and controlled crystallization are alsogiven in Table I. The following meanings apply

-   -   T_(N) and t_(N) Temperature and time used for nucleation    -   T_(C) and t_(c) Temperature and time used for crystallization of        lithium disilicate or lithium metasilicate

It can be seen that a first heat treatment in the range of from 480 to510° C. resulted in the formation of lithium silicate glasses withnuclei and these glasses crystallized in the case of Examples 1-10 and12 by a further heat treatment already at 700 to 750° C. and inparticular 700° C. to glass ceramics with lithium disilicate as maincrystal phase, as was established by X-ray diffraction tests. The heattreatment at a temperature of only 660 to 680° C. resulted in the caseof Examples 11 and 13-16 in the formation of glass ceramics with lithiummetasilicate as main crystal phase.

The produced lithium disilicate glass ceramics had high fracturetoughness values, measured as critical stress intensity factor K_(IC),of more than 2.0 MPa·m^(0.5).

The biaxial strength GB was also high, at at least 480 MPa. It wasdetermined according to dental standard ISO 6872 (2008) on test piecesthat were prepared by machining of the respective lithium disilicateglass ceramic. A CEREC-InLab machine (Sirona, Bensheim) was used formachining.

The produced lithium disilicate glass ceramics and lithium metasilicateglass ceramics were able to be very satisfactorily machined in a CAD/CAMprocess or hot pressed into the form of various dental restorations,which were also provided with a veneer if required.

They were also able to be applied by hot pressing as coatings onto inparticular dental restorations, e.g. in order to veneer the latter asdesired.

Example 17 Hot Pressing of Glass with Nuclei

A glass with the composition according to Examples 6 and 7 was preparedby mixing corresponding raw materials in the form of oxides andcarbonates for 30 min in a Turbula mixer and then melting the mixture at1450° C. for 120 min in a platinum crucible. The melt was poured intowater in order to obtain a finely divided glass granulate. This glassgranulate was melted again at 1530° C. for 150 min in order to obtain aglass melt with particularly high homogeneity. The temperature wasreduced to 1500° C. for 30 min and cylindrical glass blanks with adiameter of 12.5 mm were then prepared by pouring into pre-heated,separable steel moulds or graphite moulds. The obtained glass cylinderswere then nucleated in the range of from 480-560° C., depending on thecomposition, and stress-relieved.

The nucleated glass cylinders were then processed by hot pressing at apressing temperature of 900-1100° C. using an EP600 press furnace,Ivoclar Vivadent AG, to form dental restorations, such as inlays,onlays, veneers, partial crowns, crowns, laminating materials andlaminates. In each case, lithium disilicate was detected as main crystalphase.

TABLE I Example 1 2 3 4 5 6 7 8 9 10 11 12 Composition wt.-% wt.-% wt.-%wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% SiO₂ 73.8 75.073.8 72.8 73.8 73.8 73.8 73.8 75.3 76.6 62.1 76.4 Li₂O 15.3 15.5 15.315.1 15.3 15.3 15.3 15.3 15.7 15.9 12.9 15.9 P₂O₅ 3.4 3.4 3.4 2.9 3.43.4 3.4 3.4 3.4 — 7.0 3.5 Al₂O₃ 3.0 3.0 3.0 3.2 — — — — 3.5 3.0 3.0 —Rb₂O 4.5 3.1 — 6.0 4.5 — — 2.0 2.0 4.5 7.5 4.2 Cs₂O — — 4.5 — — 4.5 4.5— — — 7.5 — Y₂O₃ — — — — 3.0 3.0 — 3.0 — — — — TiO₂ — — — — — — 3.0 — —— — — MgO — — — — — — — 2.5 — — — — Ag₂O 0.1 — — — Optical propertiestrans- trans- trans- trans- trans- trans- trans- trans- trans- trans-slightly trans- (after pouring) parent parent parent parent parentparent parent parent parent parent opalescent parent T_(g)/° C. 467 475470 469 479 481 479 471 471 471 488 475 T_(N)/° C. 480 480 480 500 500500 500 500 490 490 510 500 t_(N)/min. 10 10 10 10 10 10 10 10 10 10 1010 T_(C)/° C. 700 700 700 700 700 700 700 700 700 750 680 740 t_(C)/min.20 20 20 20 20 20 20 20 20 30 20 20 Main crystal lithium lithium lithiumlithium lithium lithium lithium lithium lithium lithium lithium lithiumphase di- di- di- di- di- di- di- di- di- di- meta- di- silicatesilicate silicate silicate silicate silicate silicate silicate silicatesilicate silicate silicate Other crystal Li₂SiO₃ Li₃PO₄ Li₂SiO₃ quartz,Li₃PO₄ Li₃PO₄ Li₃PO₄ Li₃PO₄ Li₂SiO₃ lithium — Li₃PO₄, phases Li₂SiO₃meta- cristo- silicate balite K_(IC)/MPa · m^(1/2) 2.29 2.31 2.08 2.672.38 2.45 2.56 2.36 — — — — σ_(B)/MPa 610 — — — — 480 — — — — — —Example 13 14 15 16 Composition wt.-% wt.-% wt.-% wt.-% SiO₂ 70.1 72.870.1 70.2 Li₂O 14.5 15.1 19.0 14.5 P₂O₅ 3.2 4.0 3.4 3.2 Al₂O₃ 2.9 3.03.0 2.8 Rb₂O 4.3 5.1 4.5 — Cs₂O — — — 4.3 Y₂O₃ — — — — TiO₂ — — — — MgO— — — — Ag₂O — — — — ZrO₂ 5.0 — — 5.0 Optical properties trans- trans-trans- trans- (after pouring) parent parent parent parent T_(g)/° C. 484469 455 493 T_(N)/° C. 500 500 500 500 t_(N)/min. 10 10 10 10 T_(C)/° C.660 680 660 660 t_(C)/min. 20 20 20 20 Main crystal lithium lithiumlithium lithium phase_(RT-XRD) meta- meta- meta- meta- silicate silicatesilicate silicate Other crystal — — lithium lithium phases di- di-silicate silicate

1. Lithium silicate glass ceramic which comprises a monovalent metaloxide selected from Rb₂O, Cs₂O and mixtures thereof and comprises lessthan 5.1 wt.-% Al₂O₂ and less than 1.0 wt.-% K₂O.
 2. Glass ceramicaccording to claim 1, which comprises less than 3.8 wt.-% BaO.
 3. Glassceramic according to claim 1, which comprises K₂O, Na₂O and mixturesthereof in an amount of less than 1.0 wt.-%.
 4. Glass ceramic accordingto claim 1, wherein the molar ratio of the monovalent metal oxide toAl₂O₂ is at least 0.5.
 5. Glass ceramic according to claim 4, whereinthe molar ratio of the monovalent metal oxide to Al₂O₂ is 0.5 to 1.5. 6.Glass ceramic according to claim 1, which comprises the monovalent metaloxide or mixtures thereof in an amount of from 0.1 to 17.0 wt.-%. 7.Glass ceramic according to claim 6, which comprises the monovalent metaloxide or mixtures thereof in an amount of from 1.0 to 15.0 wt.-%. 8.Glass ceramic according to claim 7, which comprises the monovalent metaloxide or mixtures thereof in an amount of from 1.5 to 8.0 wt.-%. 9.Glass ceramic according to claim 1, which comprises 55.0 to 85.0 wt.-%SiO₂.
 10. Glass ceramic according to claim 1, which comprises 9.0 to20.0 wt.-% Li₂O.
 11. Glass ceramic according to claim 1, wherein themolar ratio between SiO₂ and Li₂O is from 2.2 to 2.6.
 12. Glass ceramicaccording to claim 1, which comprises 0 to 12.0 wt.-% P₂O₅.
 13. Glassceramic according to claim 1, which comprises at least one andpreferably all of the following components: Component wt.-% SiO₂ 55.0 to85.0  Li₂O 9.0 to 17.0 Rb₂O and/or Cs₂O 0.1 to 15.0 P₂O₅  0 to 12.0.


14. Glass ceramic according to claim 1, wherein lithium silicate glassceramic is excluded which comprises at least 6.1 wt.-% ZrO₂.
 15. Glassceramic according to claim 1, wherein lithium silicate glass ceramic isexcluded which comprises at least 8.5 wt.-% transition metal oxideselected from the group consisting of oxides of yttrium, oxides oftransition metals with an atomic number from 41 to 79 and mixtures ofthese oxides.
 16. Glass ceramic according to claim 1, which compriseslithium metasilicate as a main crystal phase.
 17. Glass ceramicaccording to claim 1, which comprises more than 5 vol.-% lithiummetasilicate crystals.
 18. Glass ceramic according to claim 1, whichcomprises more than 10 vol.-% lithium metasilicate crystals.
 19. Glassceramic according to claim 1, which comprises more than 20 vol.-%lithium metasilicate crystals.
 20. Glass ceramic according to claim 1,which comprises lithium disilicate as a main crystal phase.
 21. Glassceramic according to claims 1, which comprises more than 10 vol.-%lithium disilicate crystals.
 22. Glass ceramic according to claims 1,which comprises more than 20 vol.-% lithium disilicate crystals. 23.Glass ceramic according to claims 1, which comprises more than 30 vol.-%lithium disilicate crystals.
 24. Glass ceramic according to claim 1,which has lithium disilicate as a main crystal phase and a fracturetoughness, measured as KIC value, of at least about 2.0 MPa·m0.5. 25.Starting glass, which comprises the components of the glass ceramicaccording to claim
 1. 26. Lithium silicate glass with nuclei which aresuitable for forming lithium metasilicate and/or lithium disilicatecrystals, wherein the glass comprises the components of the glassceramic according to claim
 1. 27. Glass ceramic according to claim 1wherein the glass ceramic is present in the form of a powder, a granularmaterial, a blank or a dental restoration.
 28. Process for thepreparation of the glass ceramic according to claim 1, wherein astarting glass, a glass with nuclei or a glass ceramic with lithiummetasilicate as a main crystal phase is subjected to at least one heattreatment in the range of from 450 to 950° C.
 29. Process according toclaim 28, wherein (a) a starting glass is subjected to a heat treatmentat a temperature of from 480 to 560° C. in order to form the glass withnuclei, and (b) the glass with nuclei is subjected to a heat treatmentat a temperature of from 700 to 750° C. in order to form the glassceramic with lithium disilicate as main crystal phase.
 30. Process ofusing the glass ceramic according to claim 1 as a dental material. 31.Process according to claim 30, wherein the glass ceramic is shaped bypressing or machining to a dental restoration.